1. Field of the Invention
The present invention relates to a printing apparatus and method for a printer, a copy machine, facsimile terminal equipment, or the like, and specifically, to correction of the deviation of a printed position resulting from an error in transportation of a printing sheet.
2. Description of the Related Art
Conventional printing apparatuses such as printers, copy machines, and facsimile terminal equipment are equipped with a mechanism which transports a printing sheet as a printing medium. The mechanism includes a transportation roller, a pinch roller pressing the printing sheet against the transportation roller and holding the printing sheet between the pinch roller and the transportation roller, a device for causing the pinch roller to apply pressing force on the printing sheet, and other devices. Such transportation mechanism executes transporting operation for the printing sheet fed by a sheet feeding section, in a printing area by a printing head, and two pairs of such transportation mechanisms are generally provided before and behind the printing area, respectively. Thus, the printing sheet is precisely transported in the printing area, and during the transportation, predetermined tension is applied to the printing sheet to keep it flat over a wide area.
FIG. 12 is a sectional view mainly showing the transporting mechanism for the printing sheet in a conventional example of a printing apparatus based on an ink jet method.
In the figure, a printing head 7 mounted in a carriage portion 5 executes a scanning operation in a direction perpendicular to the drawing sheet, and during the scanning operation, ejects ink for performing a printing operation. In relation to the printing area covered by the printing head, a printing sheet P is transported, under the carriage portion 5, from right to left in the figure while substantially maintaining its horizontal orientation. More specifically, as the above-stated two pairs of transportation mechanisms, a pair of a transportation roller (hereinafter referred to as xe2x80x9cLF rollerxe2x80x9d) 36 and a pinch roller 37 is provided at an upstream side of the printing area, in which the printing sheet is transported, and a pair of a sheet discharging roller 41 and a spur 42 is provided at a downstream side of the printing area. Among these rollers, the pinch roller 37 is rotatably supported on a rotation shaft provided in a pinch roller holder 30. The pinch roller holder 30 is urged by a pinch roller spring 31 so that the pinch roller 37 can be pressed against the transportation roller 36. A pressing mechanism (not shown) similarly applies pressing force which is applied between the sheet discharging roller 41 and the spur 42. Thus, the print sheet is sandwiched between these two pairs of rollers. A motor (not shown) rotates the transportation roller 36, and rotationally drives the sheet discharging roller 41, which operates in connection with the transportation roller 36 via a predetermined gear train. Thus, the print sheet is transported a predetermined amount each time the print head performs a single scanning operation.
However, if the above-described transportation mechanism is used to transport the print sheet P, when the back end of the print sheet P slips out from between the transportation roller 36 and the pinch roller 37, the urging force of the pinch roller causes the print sheet P to be fed in the transporting direction. At this time, the LF roller and the sheet discharging roller may rotate a distance corresponding to the backlash of a gear train that drives these rollers, thereby causing the print sheet to be transported a distance larger than the intended predetermined value. In this case, the print head deviates from its regular position relative to the print sheet P, so that ink dots, formed on the print sheet P by ink ejected from the print head, may deviate from their intended positions. As a result, printed images and the like may be degraded.
FIGS. 13A and 13B show the positional relationship between the transportation roller 36 and the pinch roller 37. As shown in FIG. 13B, the transportation roller 36 has a length corresponding to the width of the transported print sheet P, whereas the pinch roller 37 comprises a plurality of shorter rollers disposed correspondingly to the transportation roller. With this construction, when the back end of the print sheet P slips out from between the transportation roller 36 and the pinch roller 37, the pinch roller 37 moves toward the transportation roller a distance corresponding to the thickness of the print sheet P, which has been sandwiched between the transportation roller 36 and the pinch roller 37. The urging force of the pinch roller associated with this movement causes the print sheet P to be transported an extra distance. Consequently, the print sheet P is transported a distance larger than the predetermined value. At the same time, the transportation roller rotates a corresponding distance.
To deal with such transportation errors, for example, a brake may be provided for rotation of the transportation roller so as to restrain the print sheet P from being transported an extra distance when slipping out from between the rollers. In this case, however, load torque required to drive the transportation roller increases, thus requiring the drive motor to be upgraded or the speed of transportation to be sufficiently increased.
The present invention is provided to solve these problems, and it is an object thereof to provide a printing apparatus and method which can promptly and properly correct the deviation of an image printed position caused by the behavior of a print sheet exhibited when its back end slips out from between a pair of rollers of a transportation means during transportation.
Thus, the present invention can have the following configuration:
A first aspect of the present invention is a printing apparatus having printing means that executes printing on a print medium transported along a transportation path, the apparatus being characterized by comprising upstream transporting means including a pair of opposite rollers arranged upstream of the printing means in the transportation path for transporting the print medium by rotating while sandwiching the print medium, downstream transporting means arranged downstream of the printing means in the transportation path for transporting the print medium, and storage means for storing nip position information representative of the position of a nip portion between the pair of rollers within the transportation path, the nip portion sandwiching an end of the print medium between the rollers.
Furthermore, a second aspect of the present invention is a printing method for executing printing on a print medium transported along a transportation path by using printing means, the printing method comprising the steps of transporting the print medium by upstream transporting means including a pair of opposite rollers arranged upstream of the printing means in the transportation path while sandwiching the print medium, transporting the print medium by downstream transporting means arranged downstream of the printing means in the transportation path, and storing nip position information representative of the position of a nip portion between the pair of rollers within the transportation path, the nip portion sandwiching an end of the print medium between the rollers.
With the above construction, according to the present invention, the storage means stores, as unique values for the printing apparatus, the accurate position of the nip between the pair of rollers of the transporting means for transporting the print medium while sandwiching it between the rollers. Accordingly, in a printing operation, this positional information can be used to promptly and precisely determine whether or not the back end of the print medium has slipped out from the nip portion, thereby allowing image corrections or the like to be executed on the back end of the print medium if it has slipped out from the nip portion. Consequently, high-grade printing results are obtained from all printing apparatuses without any variations. This further eliminates the need to improve transportation accuracy for print media by using a brake or the like to exert load torque on the transporting means, thereby providing an inexpensive small-sized printing apparatus.
The above and other objects, effects, features, and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.